The present invention relates to a method for mounting at least one rod lens in the interior of a tube belonging to an endoscope by immobilizing the rod lens in the interior of the tube at at least two points spaced axially apart from one another, having the following steps:
Selecting the outside diameter of the rod lens such that after introduction of the rod lens into the tube, an air gap is created circumferentially between the outer side of the rod lens and the inner side of the tube; and PA0 Positioning the outer side of the rod lens at a defined radial spacing from the inner side of the tube by way of at least two centering aids that engage at at least two points spaced axially apart from one another on the outer side of the rod lens, each centering aid having centering elements that are distributed uniformly around the circumference of the rod lens.
The invention further concerns an endoscope having a tube in whose interior at least one rod lens is immobilized at at least two points spaced axially apart from one another, the rod lens having an outside diameter which is dimensioned such that after introduction of the rod lens into the tube, an air gap is present between the outer side of the rod lens and the inner side of the tube; and the outer side of the rod lens being arranged at a defined radial spacing from the inner side of the tube.
A method of this kind, and a corresponding endoscope, are known from DE 34 31 631 C2.
Endoscopes are used generally in surgical practice to inspect body cavities and hollow organs. Endoscopes have an elongated tubular shaft in which components of an optical observation system are arranged. Also provided in the interior of the shaft are further devices, usually a light delivery device and optionally ducts for instruments, flushing liquids, or the like.
Light delivery is usually accomplished via light-conveying glass fibers.
The optical system of rigid endoscopes is usually received in a tube located inside the endoscope shaft. The essential constituents of the optical system are the lenses, rod lenses of the so-called Hopkins optical type being used, in particular, in endoscopes. The lenses and further constituents of the optical system, such as aperture stops, filters, and the like, are arranged along the tubular shaft axis; their task is to reproduce a clear image, with high resolution and high contrast, of as large a field of view as possible.
The most important prerequisite for this is precise arrangement of the lenses and the further components of the optical system with respect to one another in the interior of the tube belonging to the endoscope. The relative position of a lens is not variable, but precisely defined. Any shifting of the lenses relative to one another results in diminished image quality. Precise arrangement is often achieved by the fact that the outside diameter of a rod lens corresponds exactly to the open inside diameter of the tube, i.e. the lens is received without radial clearance.
Spacer sleeves, including a distal and a proximal terminating piece, placed between the lenses ensure axial positioning and immobilization of the lenses and optical components.
One problem is the fact that shocks occur when the endoscope is handled and especially when it is cleaned (for example during ultrasonic cleaning), causing the lenses to shift radially within the existing clearance and, in particular, causing abrasion at the contact surfaces between the outer side of the rod lenses and the inner side of the internal tube which adversely affects the optical imaging quality.
Endoscope shafts are very long as compared to their diameter: usual diameters are on the order of a few millimeters, and lengths on the order of several tens of centimeters. When rigid endoscopes are handled, bending torques act on the thin, long shafts and can cause breakage of the relatively long rod lenses made of glass materials.
One approach to immobilizing rod lenses in the interior of an endoscope while retaining a certain flexibility is described in U.S. Pat. No. 4,148,550, which corresponds to DE Unexamined Application 28 12 369.
The rod lenses provided therein are joined to one another in the interior of the endoscope by an additional inner tube. This inner tube has longitudinally extending slits in the region in which it bridges the ends of two adjacent rod lenses that are spaced apart from one another.
The rod lenses sit tightly, without radial clearance, in the inner tube, i.e. the outside diameter of a lens corresponds to the open inside diameter of the inner tube. The rod lenses are either simply jammed in place in the tube, or are bonded using adhesive. It is proposed for this purpose to use, for example, an adhesive that can be activated by heat treatment or UV radiation.
The ability to bend the tubular shaft within certain limits without thereby breaking the rod lenses is achieved by way of the slits of the inner tube. The inner tube can buckle slightly in the region of the slits, i.e. in the region between two adjacent rod lenses.
Because bending is possible only in the region of the slits of the inner tube, the minimum possible bending radius is very large. This is true in particular if the spacings between the rod lenses are very small compared to the rod lens length. Uniformly curved bending of the tubular shaft is not possible. If bending torques act directly in the region of the rod lenses and result in bending of the outer endoscope shaft, the rod lenses can still break. Bending torques of this kind acting in single-point fashion can occur if an endoscope strikes against an edge or is dropped while being handled.
DE 44 38 511 A1 (which corresponds to German utility model DE 94 17 262 U) describes an endoscope in which damage to the rod lenses is to be prevented by surrounding them coaxially, inside the tube, with a reinforcing member that, in contrast to the rod lens, is produced from a high-strength material such as metal or ceramic. The reinforcing member is shorter than the rod lens itself. This once again results in the difficulty that as the length of the reinforcing member increases, uniform curved bending of the tubular shaft is prevented. But the shorter the reinforcing member, the less stable the mounting of the rod lens inside the tube in terms of tilting. The purpose of the reinforcing lens is also not to increase the radial spacing between the rod lens and the tube. Instead, the reinforcing member is intended to protect the central region of the rod lens, in which stress is concentrated when a bending load is applied.
DE 34 31 631 C2, already mentioned above, describes a rod lens mount in which the rod lenses are mounted inside the tube by way of two spacer rings spaced axially apart from one another. As a result, the rod lenses are located at a radial spacing from the inner surface of the tube. This spacing creates a clearance, so that in response to a bending stress the tube can bend independently of the rod lens. According to this document, however, the spacer rings are made of materials such as, for example, metal, in particular brass, or plastic, in particular hard plastic and the like. Because hard materials of this kind are used, the rod lenses are very rigidly joined to the tube despite the existence of a radial spacing. Correspondingly, bending loads of the tube are also transferred, largely un-damped, to the rod lenses. Damage to the rod lenses due to bending torques is therefore not reliably prevented despite the existence of the radial spacing.
It is known from a publication in OE Reports No. 67, July 1989, page 13, published by SPIE, to adhesively bond lenses in a mount using centering elements, in the form of centering screws, that are to be removed after adhesive bonding.
DE 37 30 094 C2 has disclosed centering elements that are distributed uniformly over the circumference of a lens.
DE-A-722 587 has disclosed two centering aids spaced axially apart from one another, and DD 264 523 A1 has disclosed methods for coaxial adhesive bonding-in of lenses.
Against the background of this existing art, it is the object of the present invention to create a method and an endoscope of the type cited initially in which any transfer of bending forces to the at least one rod lens is reduced even further, while at the same time the rod lens is immobilized in secure and correctly positioned fashion in the interior of the tube.